One method for using a spray gun on an automated production line is to mount the spray gun on a programmable industrial robot. Robots have several advantages when used for spray painting. The flexibility of the robot arm allows coating highly irregular shapes such as vehicle bodies. The robot arm is capable of following the irregular surfaces of a vehicle body and also is capable of reaching into the interior of the body. Also, the robot may be programmed to coat successive articles having different configurations and it is readily programmable to coat articles different from previously coated articles.
Normally, a wrist is attached to the end of the robot arm for mounting the spray gun. The arm moves to position the spray gun in space and the wrist provides two or three axes of motion for aiming the spray gun relative to the end of the arm. However, there is a limit to the weight which can be mounted on the arm and wrist without interfering with operation of the robot. Also, it is desirable to limit the overall size of the spray gun. In the past, many robot mounted spray guns have been unnecessarily large and have been difficult to service. The robot mounted spray guns are not easily removed from the robot and have sometimes required excessive system down time for repairs. Ideally, the spray gun is easily removed from the robot wrist and replaced to minimize down time. The removed gun then is serviced without unnecessary delay to production.
Remote actuated spray guns used in automated spray systems have traditionally been activated using solenoid controlled air valves triggered by a programmable controller for the system. The solenoid valve is usually found outside the spray booth and is located in an electro-pneumatic cabinet. From the cabinet, relatively long air lines connect to the spray gun. This design concept can cause significant delays in gun triggering time because of the capacitance of the air lines. This may be particularly important during a color change cycle when the gun may be triggered on and off several times while purging paint from the gun and its supply lines.
When a spray gun is used on a programmable spray painting robot, finite control of both the air and the fluid must be established. Robots may move, for example, at a normal speed of four feet per second. This converts to a spray gun movement of approximately 2.5 inches in 50 milliseconds. Therefore, if the solenoid actuated pneumatic trigger valve is located at a considerable distance from the spray gun, long delays with accompanying long lead distances are inherent in the system. Additionally, it is common practice in the automotive industry to program a robots' painting path and triggering points within a teaching spray booth. The program is then uploaded to the actual paint spray booths. Each paint spray booth may have different locations for the pneumatic solenoid control valves. This usually means that the hose lengths between the gun and the control valves are different lengths. These different lengths cause different response times between the valves and the gun.
A single air line is commonly connected to the spray gun for supplying atomization air and pattern shaping air to the gun nozzle. The gun may include separate valves for controlling atomization air and pattern shaping air. However, these valves are not remotely adjustable by the system controller. At best, the system controller can independently turn on and off the pattern shaping air valve to select either a round spray pattern or a fan pattern. The size of the fan pattern must be manually adjusted at the spray gun.